1. Field of the Invention
The present invention relates to a plating bath which is employed for electroless bismuth plating.
2. Description of the Background Art
Electroless plating is carried out through parallel reaction of cathodic deposition of a metal and anodic oxidation of a reducing agent. In general, the reducing agent is prepared from sodium hypophosphite, formalin, sodium borohydride or dimethylamine borane to put electroless plating into practice with a metal such as nickel or cobalt.
In order to cause a plating reaction in such electroless plating, the reversible potential of the deposition metal electrode must be "nobler" than the oxidation-reduction potential of the reducing agent in terms of equilibrium. In this point, bismuth is conceivably capable of plating deposition since the same has a sufficiently "nobler" reversible potential of +0.314 V (vs. N. H. E.) than that of -0.236 V or -0.287 V (vs. N. H. E.) of nickel or cobalt, which is an element capable of carrying out electroless plating with the aforementioned reducing agent of sodium hypophosphite or the like.
However, the possibility of electroless plating is remarkably influenced by the anodic oxidation velocity of the reducing agent, which extremely depends on the electrode metal. This is because the deposition metal, which gradually covers the basis material, itself must have sufficient catalytic activity with respect to oxidation of the reducing agent in order to attain stationary progress of the plating. While nickel and cobalt are transition elements, bismuth is a typical element. It is known that a typical element has low catalytic activity or acts as a catalyst poison due to the state of its electron configuration. Thus, it has heretofore been regarded impossible to form a bismuth film by electroless plating (refer to "Nikkei Hi-Tech Information" Jun. 2, 1986, pp. 24 to 28, for example).